Method and apparatus for dynamically allocating upstream bandwidth in passive optical networks

ABSTRACT

One embodiment of the present invention provides a system that facilitates dynamic allocation of upstream bandwidth in a passive optical network which includes a central node and at least one remote node. Each remote node is coupled to at least one logical entity and includes a number of queues, each of which is associated with a logical entity and stores upstream data from the logical entity. The central node is coupled to an external network through a shared out-going uplink. During operation, the system receives a request from a remote node for a grant to transmit upstream data from a logical entity, wherein the request reports the state of a queue associated with a logical entity; wherein the size of the data to be transmitted does not exceed a transmission threshold assigned to that logical entity, and wherein a logical entity may not request more than what is allowed by the corresponding transmission threshold. If the request satisfies a bandwidth allocation policy, the system issues a grant to the remote node to transmit upstream data. In response to the grant, the system receives upstream data from the remote node. Next, the system transmits the received upstream data to the out-going uplink according to a set of SLAs.

FIELD OF THE INVENTION

The present invention relates to the design of passive optical networks.More specifically, the present invention relates to a method andapparatus for dynamically allocating upstream bandwidth in a passiveoptical network.

RELATED ART

In order to keep pace with increasing Internet traffic, optical fibersand associated optical transmission equipment have been widely deployedto substantially increase the capacity of backbone networks. However,this increase in the capacity of backbone networks has not been matchedby a corresponding increase in the capacity of access networks. Evenwith broadband solutions, such as digital subscriber line (DSL) andcable modem (CM), the limited bandwidth offered by current accessnetworks creates a severe bottleneck in delivering high bandwidth to endusers.

Among different technologies, Ethernet passive optical networks (EPONs)appear to be the best candidate for next-generation access networks.EPONs combine the ubiquitous Ethernet technology with inexpensivepassive optics. Therefore, they offer the simplicity and scalability ofEthernet, and the cost-efficiency and high capacity of passive optics.In particular, due to the high bandwidth of optical fibers, EPONs arecapable of accommodating broadband voice, data, and video trafficsimultaneously. Such integrated service is difficult to provide with DSLor CM technology. Furthermore, EPONs are more suitable for InternetProtocol (IP) traffic, since Ethernet frames can directly encapsulatenative IP packets with different sizes, whereas ATM passive opticalnetworks (APONs) use fixed-size ATM cells and consequently requirepacket fragmentation and reassembly.

Typically, EPONs are used in the “first mile” of the network, whichprovides connectivity between the service provider's central offices andbusiness or residential subscribers. Logically, the first mile is apoint-to-multipoint network, with a central office servicing a number ofsubscribers. A tree topology can be used in an EPON, wherein one fibercouples the central office to a passive optical splitter, which dividesand distributes downstream optical signals to subscribers and combinesupstream optical signals from subscribers (see FIG. 1).

Transmissions within an EPON are typically performed between an opticalline terminal (OLT) and optical networks units (ONUs) (see FIG. 2). TheOLT generally resides in the central office and couples the opticalaccess network to the metro backbone, which is typically an externalnetwork belonging to an ISP or a local exchange carrier. The ONU can belocated either at the curb or at an end-user location, and can providebroadband voice, data, and video services.

Communications within an EPON can be divided into upstream traffic (fromONUs to OLT) and downstream traffic (from OLT to ONUs). Because of thebroadcast nature of Ethernet, the downstream traffic can be deliveredwith considerable simplicity in an EPON: packets are broadcast by theOLT and extracted by their destination ONU based on their Logical LinkIdentifier (LLID). However, in the upstream direction, the ONUs need toshare the channel capacity and resources. Moreover, the burstiness ofnetwork traffic and the requirement of different service levelagreements (SLAs) make the upstream bandwidth allocation a challengingproblem.

Hence, what is needed is a method and apparatus for dynamicallyallocating upstream bandwidth in an EPON, which is fair, efficient, andresponsive, and which accommodates bursty traffic while satisfying SLAs.

SUMMARY

One embodiment of the present invention provides a system thatfacilitates dynamic allocation of upstream bandwidth in a passiveoptical network which includes a central node and at least one remotenode. Each remote node is coupled to at least one logical entity, whichcorresponds to a device or a user. Each remote node includes a number ofqueues, each of which is associated with a logical entity and storesupstream data from the logical entity. The central node is coupled to anexternal network outside of the passive optical network through a sharedout-going uplink.

During operation, the system receives a request from a remote node for agrant to transmit upstream data from a logical entity associated withthe remote node to the central node, wherein the request reports thestate of a queue associated with a logical entity; wherein the size ofthe data to be transmitted does not exceed a transmission thresholdassigned to that logical entity, and wherein a logical entity may notrequest more than what is allowed by the corresponding transmissionthreshold. If the request satisfies a bandwidth allocation policy, thesystem issues a grant to the remote node to transmit upstream data. Inresponse to the grant, the system receives upstream data from the remotenode. Next, the system transmits the received upstream data to theout-going uplink according to a set of SLAs.

In a variation of this embodiment, the system stores the state of thequeue associated with the logical entity. The system then reads thestate of the queue associated with the logical entity into a hardwarescheduler. Next, the system schedules a grant in response to the requestat the hardware scheduler and writes the grant to a grant buffer fromthe hardware scheduler. The system then transmits the grant to theremote node.

In a variation of this embodiment, the system stores the state of thequeue associated with the logical entity. The system then reads thestate of the queue associated with the logical entity into aprogrammable processor. Next, the system schedules a grant in responseto the request at the hardware scheduler and writes the grant to a grantbuffer from the programmable processor. The system then transmits thegrant to the programmable processor.

In a further variation, the system prevents a hardware scheduler fromwriting a grant into the grant buffer when the programmable processorwrites to the grant buffer.

In a further variation, the programmable processor is programmed toimplement one or more scheduling policies.

In a further variation, the programmable processor is an on-boardprocessor which resides with a hardware scheduler on a circuit board.

In a further variation, the programmable processor is an externalprocessor which resides outside a circuit board where a hardwarescheduler resides.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a passive optical network wherein a central officeand a number of subscribers form a tree topology through optical fibersand a passive optical splitter (prior art).

FIG. 2 illustrates a passive optical network including an OLT and ONUs(prior art).

FIG. 3 illustrates the architecture of an OLT that facilitates dynamicupstream bandwidth allocation in accordance with an embodiment of thepresent invention.

FIG. 4 presents a flow chart illustrating the dynamic upstream bandwidthallocation process in accordance with an embodiment of the presentinvention.

FIG. 5 illustrates a flow-control mechanism within an OLT thatfacilitates dynamic upstream bandwidth allocation in accordance with anembodiment of the present invention.

FIG. 6 illustrates a hierarchical round-robin scheduling scheme withtransmission thresholds in accordance with an embodiment of the presentinvention.

FIG. 7 illustrates a time-out mechanism for outstanding data thatprovides fault tolerance in accordance with an embodiment of the presentinvention.

FIG. 8A illustrates a hardware-based DBA scheduler in accordance with anembodiment of the present invention.

FIG. 8B illustrates a software-based DBA scheduler in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

The data structures and code described in this detailed description aretypically stored on a computer readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. This includes, but is not limited to, application specificintegrated circuits (ASICs), field-programmable gate arrays (FPGAs),semiconductor memories, magnetic and optical storage devices such asdisk drives, magnetic tape, CDs (compact discs) and DVDs (digitalversatile discs or digital video discs), and computer instructionsignals embodied in a transmission medium (with or without a carrierwave upon which the signals are modulated). For example, thetransmission medium may include a communications network, such as theInternet.

Passive Optical Network Topology

FIG. 1 illustrates a passive optical network, wherein a central officeand a number of subscribers form a tree topology through optical fibersand a passive optical splitter. As shown in FIG. 1, a number ofsubscribers are coupled to a central office 101 through optical fibersand a passive optical splitter 102. Passive optical splitter 102 can beplaced in the vicinity of end-user locations, so that the initial fiberdeployment cost is minimized. The central office is coupled to anexternal network, such as a metropolitan area network operated by anISP.

FIG. 2 illustrates a passive optical network including an OLT and ONUs.OLT 201 is coupled with ONUs 202, 203, and 204 through optical fibersand a passive optical splitter. An ONU can accommodate a number ofnetworked devices, such as personal computers, telephones, videoequipment, network servers, etc. Note that a networked device canidentify itself by using an LLID, as defined in the IEEE 802.3 standard.

Dynamic Bandwidth Allocation Mechanism

FIG. 3 illustrates the architecture of an OLT that facilitates dynamicupstream bandwidth allocation in accordance with an embodiment of thepresent invention. In this example, an OLT 320 accepts requests andupstream data traffic from ONUs 301 and 302. Each ONU maintains a numberof queues, for example queues 311, 312, and 313, each of which storesupstream data from an LLID corresponding to a device or a user thatcouples to that ONU. Note that upstream data from an LLID is carried indata frames (e.g., Ethernet frames), which have variable sizes. Duringtransmission these data frames are removed from their respective queue.An LLID requests a grant, to transmit upstream data, via a reportmessage. The report message indicates the amount of data in the LLID'scorresponding queue(s). Typically, these request messages can piggybackon an upstream data transmission.

Within OLT 310, a dynamic bandwidth allocation (DBA) scheduler 303receives the report messages from ONUs. OLT 310 also includes a FIFOqueue controller (FCT) 305, which contains a number of FIFO queues (321,322, 323, 324, and 325) that are associated with different LLIDs.Upstream data from each LLID is temporarily stored in these FIFO queuesbefore being transmitted to the external ISP network through a shareduplink 330. The state of these FIFO queues is monitored and stored in aqueue length table 304.

After receiving a request from an LLID, DBA scheduler 303 determineswhether a grant to transmit can be sent to the requesting LLID based ontwo considerations. First, whether there is sufficient available spacein the FIFO queue corresponding to the requesting LLID, according queuelength table 304. Second, whether the requesting LLID is the next inturn to transmit data as scheduled. (Note that proper scheduling ofLLIDs for upstream data transmission is necessary to guarantee fair andefficient bandwidth allocation among all the LLIDs.) When bothconditions are met, the DBA scheduler issues a grant to the requestingLLID. The grant allocates an upstream transmission time slot to theLLID.

Note that outstanding data for each LLID can be taken into account inthe calculation of available space in the FIFO queues. Outstanding datais the “in-flight” data for which a grant for transmission has beengiven, but which has not been received by OLT 320. Records ofoutstanding data are stored in data structure 309. When calculatingavailable space in a FIFO queue, DBA scheduler 303 subtracts the amountof outstanding data of the requesting LLID from the available physicalspace in the corresponding FIFO queue, and uses the result as the actualavailable space for future data transmission.

With regard to scheduling upstream transmission, one possible scheme isthe hierarchical round-robin scheme, which can be used to fairly andefficiently allocate bandwidth among all LLIDs. Another possiblescheduling scheme is strict priority scheduling. However, because SLAsusually place constraints on parameters such as average bit rate,maximum delay, etc., a transmission threshold (the maximum amount ofdata in each transmission) may be set for every LLID in the hierarchicalround-robin scheme. A more detailed discussion of this scheme appears inthe discussion related to FIG. 5 below.

OLT 320 further includes a bandwidth shaper 307, which retrieves datastored in the FIFO queues within FCT 305 and transmits the retrieveddata to shared uplink 330. Bandwidth shaper 307 ensures that the datastored in FCT 305 is served in accordance with the priorityclassification and SLA pertinent to each LLID, which is stored in datastructure 306. Like the scheduling mechanism within DBA scheduler 303,the scheduling mechanism within bandwidth shaper 307 is desired to befair and efficient, and therefore can also use the hierarchicalround-robin scheduling scheme.

FIG. 4 presents a flow chart illustrating the dynamic upstream bandwidthallocation process in accordance with an embodiment of the presentinvention. The system starts by receiving a report message from an LLIDat the DBA scheduler 303 (step 401). DBA scheduler 303 then determinesif there is sufficient space in the FIFO queue within FCT 305 for thisLLID (taking into account the outstanding data) (step 402). If there isnot sufficient space, DBA scheduler temporarily holds the grant for therequesting LLID until sufficient space becomes available in the FIFOqueue. Meanwhile, the system can receive and process requests from otherLLIDs by returning to step 401.

If there is sufficient space in the FIFO queue within FCT 305, DBAscheduler 303 further determines if the requesting LLID is scheduled totransmit data next (step 403). If not, DBA scheduler 303 willtemporarily hold the grant until the requesting LLID is the next totransmit. Meanwhile, the system can receive and process requests fromother LLIDs by returning to step 401.

If it is the requesting LLID's turn to transmit, DBA scheduler generatesa grant and sends it to the requesting LLID (step 404). The system thenreturns to step 401 and continues to receive and process subsequentrequests.

Flow-Control Mechanism

FIG. 5 illustrates a flow-control mechanism within an OLT thatfacilitates dynamic upstream bandwidth allocation in accordance with anembodiment of the present invention. In this example, when FIFO queue323 is filled, DBA scheduler 303 stops granting transmission from LLID#3, thereby causing queue 313 to fill. ONU 302 can then generate aflow-control message in accordance with the IEEE 802.3×standard to thecorresponding device or user to slow down, or pause, further upstreamdata transmission.

Hierarchical Round-Robin Scheduling with Transmission Thresholds

FIG. 6 illustrates a hierarchical round-robin scheduling scheme withtransmission thresholds in accordance with an embodiment of the presentinvention. This hierarchical round-robin scheduling is performed asfollows:

First, group all LLIDs with the highest priority (priority 0). Withinpriority 0, assign each LLID a transmission slot in accordance to anamount of data burst the LLID is allowed to transmit upstream. The LLIDis provisioned to not report a value greater than this amount. Althoughthe aggregate of all report messages in a report frame may exceed thisthreshold, the amount of data implied in each individual message cannotexceed this burst size. The slot size provisioned for each LLID isdetermined such that all the LLIDs may be serviced within a fixed delaybounds. For example, if the delay bounds for priority 0 is one ms, andshared uplink 330's data speed is 1 Gb/s, then the total duration ofpriority 0 may not exceed 1000 Kb. Therefore, the aggregate slot size ofpriority 0 LLIDs would sum up to less than or equal to 1000 Kb.

Within priority 0, one slot is allocated for lower priority traffic.This slot is denoted as the drop-down slot. All lower-priority trafficis allowed to transmit within this reserved slot.

Next, group all of the LLIDs with the second highest priority (priority1). Within priority 1, assign each LLID a transmission slot according tothe maximum burst the LLID may transmit upstream. The LLID will beconfigured such that it will observe this maximum burst size whenreporting. A slot in priority 1 is allowed to transmit inside the slotreserved for lower-priority traffic (the drop-down slot) within priority0. Since a priority 1 LLID may only transmit when priority 0 istransmitting its drop-down slot, the delay of the queuing delay ofpriority 1 LLIDs is typically many times of the queuing delay ofpriority 0 LLIDs.

Within priority 1, there is similarly one slot reserved forlower-priority traffic.

As shown in FIG. 6, one can repeat steps similar to the above, andconstruct an entire hierarchy to accommodate all the LLIDs. Note thatthe transmission thresholds of LLIDs within a given priority level isbased on the bandwidth and maximum allowable delay negotiated in thecorresponding SLA.

Fault Tolerance

FIG. 7 illustrates a time-out mechanism for outstanding data thatprovides fault tolerance in accordance with an embodiment of the presentinvention. During operation, it is possible that a grant message 731 islost on its way from OLT 720 to ONU 610, for example due to a bit error.As a result, the subsequent grant messages received by ONU 710 for thesame LLID will grant transmission sizes that are inconsistent with theamount of data available for upstream transmission. This may manifestitself by the ONU receiving a grant that is not a frame boundary. OnceONU 710 detects this inconsistency, it will start sending special reportmessages to OLT 720, requesting a transmission size of 0 Kb. Meanwhile,OLT 720 keeps track of when a piece of upstream data associated with agrant is due to arrive. Whether or not this piece of data physicallyarrives for the grant, the OLT removes the information corresponding tothe outstanding data for the grant.

After sending the special report messages (with request of 0 K) for aperiod of time, ONU 710 resumes sending normal request messages. By thistime the lost grant message, and its residual effects, would have timedout in OLT 720 and normal operation resumes.

It is possible for an ONU to track the amount of time between grants. Ifthe amount of time between grants exceeds a certain interval, ONU 710sets an alarm and sends a message to OLT 720 via an OAM frame. This canbe done via an LLID on the ONU that is reserved for processor traffic.This message will instruct OLT 720 that an LLID is not being granted.One way for OLT 720 to deal with this situation is to reset the LLIDentry in the DBA and bandwidth shaper tables.

In another scenario, OLT 720 periodically sends out polls to ONUs to seeif an LLID has any data to send. Polls are grants for 64 bytes of datathat have a forced-report flag asserted. The only upstream datatransmitted as a response to a poll is a single report frame. Thepolling frequency reflects the SLA of an LLID. For example, the pollsfor priority 0 LLIDs are sent every 1 ms. If a grant previouslyoccurred, the subsequent poll will be sent at 1 ms after that grantbeing sent.

Correspondingly, a non-poll grant is a grant that allows transmission ofmore than just a single report frame. An ONU tracks the amount of timeelapsed between non-poll grants for each LLID. If this time exceeds acertain interval, the ONU sets an alarm. If the alarm is set, and theONU has data to send, the ONU will send a message to the OLT, via an OAMframe, denoting the error condition. This will instruct the OLT that anLLID is in an error state. One way for the OLT to deal with thissituation is to reset or modify the LLID entry in the DBA and bandwidthscheduler tables.

Hardware-Based and Software-Based DBA Scheduler

FIG. 8A illustrates a hardware-based DBA scheduler in accordance ith anembodiment of the present invention. One way to implement a DBAscheduler is to embed it in the OLT hardware, such as in an ASIC orFPGA. In general, a hardware-based DBA scheduler is fast and has a smallfootprint, but it may also lack the flexibility necessary forimplementing user-defined arbitrary scheduling policies.

As shown in FIG. 8A, in a hardware-based implementation, the receiver ofan OLT stores queue information of different ONUs in a report-queuestatus RAM 810 within the OLT. Hardware DBA scheduler 820 reads thequeue status information from report-queue status RAM 810, and schedulesgrant messages corresponding to each report message based on givenscheduling policies (e.g., SLAs). Scheduler 820 then writes the grantmessages to a buffer, such as grant FIFO buffer 830. Subsequently, thegrant messages are removed from the grant buffer and transmitted to ONUsby the transmitter.

FIG. 8B illustrates a software-based DBA scheduler in accordance with anembodiment of the present invention. Although hardware-based DBAscheduler can be fast and have small footprint, sometimes it isdesirable to have the flexibility in a DBA scheduler to implementuser-defined arbitrary scheduling policies. One way to obtain suchflexibility is to use an external programmable processor, whichfunctions as a software-based DBA scheduler.

As shown in FIG. 8B, in a software-based implementation, the receiver ofan OLT stores queue information of different ONUs in a report-queuestatus RAM 810 within the OLT. The queue status information is then readby an external programmable processor 840. Note that a user may programprocessor 840 to implement arbitrary scheduling policies. Also,processor 840 can reside on the same board as report-queue status RAM810, or can reside independently outside the OLT. Based on givenscheduling policies and the report-queue status information, processor840 schedules grant messages corresponding to each report message.Processor 840 then writes the grant messages to a buffer, such as grantFIFO buffer 830. Subsequently, the grant messages are removed from thegrant buffer and transmitted to ONUs by the transmitter.

It is possible to have both hardware-based and software-based DBAschedulers, as in the example illustrated in FIG. 8B. In this example, auser has the freedom to choose between the hardware-based DBA schedulerand the software-based programmable DBA scheduler. Typically, when asoftware-based DBA scheduler (processor 840) is writing a grant messageto grant FIFO buffer 830, hardware DBA scheduler 820 is prohibited towrite grant messages to grant FIFO buffer 830 to prevent conflicts.

The foregoing descriptions of embodiments of the present invention havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

1. A method for dynamically allocating upstream bandwidth in a passiveoptical network that includes a central node and at least one remotenode, wherein each remote node is coupled to at least one logical entitycorresponding to a device or a user; wherein each remote node includes anumber of queues, each of which is associated with a logical entity andstores upstream data from the logical entity; and wherein a sharedout-going uplink couples the central node to an external network outsideof the passive optical network; the method comprising: receiving arequest from a remote node for a grant to transmit upstream data from alogical entity associated with the remote node to the central node,wherein the request reports the state of a queue associated with alogical entity; wherein the size of the data to be transmitted does notexceed a transmission threshold assigned to that logical entity; andwherein a logical entity may not request more than what is allowed bythe corresponding transmission threshold; if the request satisfies abandwidth allocation policy, issuing a grant to the remote node totransmit upstream data; in response to the grant, receiving upstreamdata from the remote node; and transmitting the received upstream datato the out-going uplink according to a set of service level agreements.2. The method of claim 1, further comprising: storing the state of thequeue associated with the logical entity; reading the state of the queueassociated with the logical entity into a hardware scheduler; schedulinga grant in response to the request at the hardware scheduler; writingthe grant to a grant buffer from the hardware scheduler; andtransmitting the grant to the remote node.
 3. The method of claim 1, themethod further comprising: storing the state of the queue associatedwith the logical entity; reading the state of the queue associated withthe logical entity into a programmable processor; scheduling a grant atthe programmable processor in response to the request; writing the grantto a grant buffer from the programmable processor; and transmitting thegrant to the remote node.
 4. The method of claim 3, further comprisingpreventing a hardware scheduler from writing a grant into the grantbuffer when the programmable processor writes to the grant buffer. 5.The method of claim 3, wherein the programmable processor is programmedto implement one or more scheduling policies.
 6. The method of claim 3,wherein the programmable processor is an on-board processor whichresides with a hardware scheduler on a circuit board.
 7. The method ofclaim 3, wherein the programmable processor is an external processorwhich resides outside a circuit board where a hardware 3 schedulerresides.
 8. An apparatus that dynamically allocates upstream bandwidthin a passive optical network, comprising: a central node; at least oneremote node, wherein each remote node is coupled to at least one logicalentity corresponding to a device or a user; and wherein each remote nodeincludes a number of queues, each of which is associated with a logicalentity and stores upstream data from the logical entity; a sharedout-going uplink that couples the central node to an external networkoutside of the passive optical network; a dynamic bandwidth allocationmechanism within the central node configured to, receive a request froma remote node for a grant to transmit upstream data from a logicalentity associated to the remote node to the central node, wherein therequest reports the state of a queue associated with a logical entity;wherein the size of the data to be transmitted does not exceed atransmission threshold assigned to that logical entity; and wherein alogical entity may not request more than what is allowed by thecorresponding transmission threshold, and if the request satisfies abandwidth allocation policy, issue a grant to the remote node totransmit upstream data; a receiving mechanism configured to receiveupstream data from the remote node in response to the grant; and abandwidth shaping mechanism configured to transmit the received upstreamdata to the out-going uplink according to a set of service levelagreements.
 9. The apparatus of claim 8, wherein the dynamic bandwidthallocation mechanism is further configured to store the state of thequeue associated with the logical entity; and wherein the apparatusfurther comprises a hardware scheduler configured to, read the state ofthe queue associated with the logical entity into a hardware scheduler;schedule a grant in response to the request at the hardware scheduler;write the grant to a grant buffer from the hardware scheduler; andtransmit the grant to the remote node.
 10. The apparatus of claim 8,wherein the dynamic bandwidth allocation mechanism is further configuredto store the state of the queue associated with the logical entity; andwherein the apparatus further comprises a programmable processorconfigured to, read the state of the queue associated with the logicalentity into a programmable processor; schedule a grant in response tothe request at the programmable processor; write the grant to a grantbuffer from the programmable processor; and transmit the grant to theremote node.
 11. The apparatus of claim 8, further comprising a hardwarescheduler prevented from writing a grant into the grant buffer when theprogrammable process writes to the grant buffer.
 12. The apparatus ofclaim 8, wherein the programmable processor is programmed to implementone or more scheduling policies.
 13. The apparatus of claim 8, whereinthe programmable processor is an on-board processor which resides with ahardware scheduler on a circuit board.
 14. The apparatus of claim 8,wherein the programmable processor is an external processor whichresides outside a circuit board where a hardware scheduler resides. 15.A computer-readable storage medium storing instructions that whenexecuted by a computer cause the computer to perform a method fordynamically allocating upstream bandwidth in a passive optical networkthat includes a central node and at least one remote node, wherein eachremote node is coupled to at least one logical entity corresponding to adevice or a user; wherein each remote node includes a number of queues,each of which is associated with a logical entity and stores upstreamdata from the logical entity; and wherein a shared out-going uplinkcouples the central node to an external network outside of the passiveoptical network; the method comprising: receiving a request from aremote node for a grant to transmit upstream data from a logical entityassociated with the remote node to the central node, wherein the requestreports the state of a queue associated with a logical entity; whereinthe size of the data to be transmitted does not exceed a transmissionthreshold assigned to that logical entity; and wherein a logical entitymay not request more than what is allowed by the correspondingtransmission threshold; if the request satisfies a bandwidth allocationpolicy, issuing a grant to the remote node to transmit upstream data; inresponse to the grant, receiving upstream data from the remote node; andtransmitting the received upstream data to the out-going uplinkaccording to a set of service level agreements.
 16. Thecomputer-readable storage medium of claim 15, wherein the method furthercomprises: storing the state of the queue associated with the logicalentity; reading the state of the queue associated with the logicalentity into a hardware scheduler; scheduling a grant in response to therequest at the hardware scheduler; writing the grant to a grant bufferfrom the hardware scheduler; and transmitting the grant to the remotenode.
 17. The computer-readable storage medium of claim 15, wherein themethod further comprising: storing the state of the queue associatedwith the logical entity; reading the state of the queue associated withthe logical entity into a programmable processor; scheduling a grant atthe programmable processor in response to the request; writing the grantto a grant buffer from the programmable processor; and transmitting thegrant to the remote node.
 18. The computer-readable storage medium ofclaim 17, wherein the method further comprises preventing a hardwarescheduler from writing a grant into the grant buffer when theprogrammable processor writes to the grant buffer.
 19. Thecomputer-readable storage medium of claim 17, wherein the programmableprocessor is programmed to implement one or more scheduling policies.20. The computer-readable storage medium of claim 17, wherein theprogrammable processor is an on-board processor which resides with ahardware scheduler on a circuit board.
 21. The computer-readable storagemedium of claim 17, wherein the programmable processor is an externalprocessor which resides outside a circuit board where a hardwarescheduler resides.